1. Field of the Invention
The present invention relates generally to a laser processing apparatus and a laser processing method for processing a workpiece such as a semiconductor wafer by using a laser beam, and more particularly to a technique for correcting the focal position of the laser beam.
2. Description of the Related Art
In a semiconductor device fabrication process, a plurality of circuits such as ICs (integrated circuits) and LSIs (large-scale integrated circuits) are formed like a matrix on the surface of a substantially disk-shaped semiconductor wafer, and this wafer thus formed with the plural circuits is next cut along a plurality of crossing streets (division lines) to thereby divide the plural circuits from each other as chips. Also in a fabrication process for optical devices such as light emitting diodes and laser diodes widely used in electrical equipment, a gallium nitride compound semiconductor or the like is formed on the surface of a substrate such as a sapphire substrate to obtain an optical device wafer, and this wafer is next cut along a plurality of division lines to divide individual devices from each other as chips.
Cutting (dicing) of such various wafers is performed by using a cutting apparatus called a dicer. As another method, there has recently been developed a method of cutting a workpiece such as a semiconductor wafer by using a laser beam (see Japanese Patent Laid-open No. Hei 10-305420 and Japanese Patent No. 3408805, for example). Japanese Patent Laid-open No. Hei 10-305420 discloses a laser processing method such that a laser beam is applied to a workpiece formed from a single crystal oxide to dissociate and evaporate the molecules of the single crystal oxide by a photochemical reaction, thereby forming a groove at a predetermined position on the workpiece. Then, the workpiece is cleaved along this groove.
Japanese Patent No. 3408805 discloses a laser cutting method such that a pulsed laser beam having a transmission wavelength to a workpiece is applied to the workpiece in the condition where the focal position of the laser beam is set inside the workpiece, thereby forming a modified layer along each division line. This modified layer is smaller in strength than the other region of the workpiece. Accordingly, by applying an external force along each division line, the workpiece is divided along each division line in such a manner that the division is started from the modified layer.
In some workpieces to be processed, there is a possibility that warpage or undulation may occur or the surface of the workpiece may not become flat to cause the formation of roughness during any process steps before a dicing step. In the case of processing such a workpiece, the surface displacement of a subject surface of the workpiece is not constant, so that the focal position of the laser beam varies in a direction along the thickness of the workpiece, causing a reduction in processing accuracy. In the laser processing method for forming a modified layer inside the workpiece as mentioned above, the effect of warpage, undulation, and roughness of the workpiece is especially profound. To cope with this problem, there has been proposed a laser processing apparatus including means for measuring the surface displacement in a laser beam applying region of the workpiece in advance, means for adjusting the focal position of a laser beam in a direction along the thickness of the workpiece according to the result of measurement by the above measuring means, and means for applying the laser beam to the workpiece (see Japanese Patent Laid-open Nos. 2007-152355 and 2008-16577, for example).
However, the laser processing apparatus described in Japanese Patent Laid-open Nos.125  2007-152355 and 2008-16577 has a problem such that when the processing position, i.e., the focal position of the processing laser beam is far from the surface of the workpiece, it is impossible to measure the surface displacement during laser processing and it is therefore impossible to correct the focal position of the processing laser beam in real time, thus causing a reduction in processing efficiency. That is, in the laser processing apparatus described in Japanese Patent Laid-open Nos. 2007-152355 and 2008-16577, the focal point of a detecting laser beam and the focal point of the processing laser beam are set at the same position. Accordingly, in the case of focusing the processing laser beam inside the workpiece at a deep position, the spot diameter of the detecting laser beam on the surface of the workpiece becomes large.
In this conventional laser processing apparatus, the surface displacement of the workpiece is detected from the light quantity of reflected light reflected on the surface of the workpiece by the application of the detecting laser beam to the workpiece. Accordingly, when the spot diameter of the detecting laser beam is large, the light quantity of the reflected light per unit area is reduced, so that the surface displacement of the workpiece cannot be accurately measured. Particularly in the case of applying a processing laser beam to a thick workpiece plural times starting at a focal position near the surface of the workpiece on the back side thereof, the distance between the focal position of the processing laser beam and the surface of the workpiece on the front side thereof becomes large, so that there is a possibility that the measurement itself of the surface displacement cannot be performed. For this reason, in the laser processing apparatus described in Japanese Patent Laid-open Nos. 2007-152355 and 2008-16577 mentioned above, the detecting laser beam is applied to the workpiece prior to laser processing to preliminarily measure the surface displacement of the workpiece. Thereafter, the focal position of the processing laser beam is adjusted to perform laser processing.